Author: Sietze Wijma

This flavour is neither the result of using green pepper extract, nor is it associated with soaking bell peppers in wine! So where does the often-mentioned aroma of green bell pepper come from?

Origins
It is widely accepted that the aromatic compound 2-isobutyl-3-methoxypyrazine is responsible for this flavour note. It is often abbreviated to IBMP and may go by the trivial name bell pepper pyrazine.
The compound is closely associated with Sauvignon Blanc and related grape varieties, and therefore considered a varietal flavour.
It is a primary flavour, i.e. a flavour originating from the grape or fermentation. The compound is found in the grape flesh and skins in its free, volatile form. Hence, when grapes contain IBMP, you would be able to notice it when tasting those grapes from the vine. Importantly, this is different from many other aroma compounds, including thiols and terpenes, which are bound to a grape component and hence odourless until released during fermentation.

Influencing factors – vineyard
Ripeness is a crucial factor. Bell pepper pyrazine peaks at veraison [2], after which it steadily declines. Vineyard temperature and sunlight exposure are also important factors. They lead to a decrease in IBMP. Training an open canopy can help lower levels of IBMP. Warmer sites, or those with slopes facing the sun typically yield grapes with lower levels of IBMP.

Influencing factors – winery
There are less option available in the winery to modulate the flavour. Juice clarification prior to fermentation reduces the compound to some extent. Oak ageing can also reduce the impact of the flavour by masking it with other flavours such as vanilla (from vanillin) and clove (from eugenol). Oak ageing, however, is not commonly practised with Sauvignon Blanc. Separate batches of wine with different levels of IBMP could be blended to modify the impact of IBMP. Other options in terms of winemaking are limited. IBMP is not very sensitive to oxidative treatments [3].

Presence elsewhere
Bell pepper pyrazine is the main constituent of the aroma of green bell peppers. In bell peppers, the compound develops in a similar way as in grapes. In green bell peppers, which are essentially the unripe version of its red counterpart, IBMP levels are high. It declines during ripening.

Descriptors
Common descriptors of wines high in IBMP include green, green bell pepper, vegetal, herbaceous, green beans, earthy, capsicum, jalapeños.

Detection threshold
In white wine, the detection threshold is about 4 nanogram / litre. That is very very little indeed. This detection threshold implies that at a concentration of 4 ng IBMP / litre wine, half of the population would notice the flavour. A nanogram equals one millionth of a milligram. To put that into perspective, a sugar granule is about 1 milligram.
Differences in sensitivity exist and the specific anosmia (=inability to smell specific compounds) rate of this compound is about 7% of the population. Don’t worry if you can’t pick up this particular compound – you may still be able to smell other compounds linked to Sauvignon Blanc, such as 4-MMP (smells of box tree or gooseberry). Other compounds linked to underripe grapes or stems include cis-3-hexenol (smells of freshly cut grass).

Other origins of methoxypyrazines
Inclusion of stems and leaves in the vinification process may increase levels of IBMP. IBMP and other methoxypyrazines are also secreted by ladybugs, causing ladybug taint in wine [4].

Other pyrazines in wine
IBMP is one of several methoxypyrazines found in wine.
Although methoxypyrazines are often shortened to simply ‘pyrazines’, they are not mutually exclusive. While all methoxypyrazines are indeed pyrazines, methoxypyrazines refer to the bell pepper pyrazine and potato pyrazine (2-isopropyl-3-methoxypyrazine). Potato pyrazine smells like potato skins or dug soil. In wine it is often at levels below the detection threshold, making this compound less relevant than its bell pepper equivalent.
Other pyrazines, not belonging to the methoxypyrazines family, are the result of the Maillard reaction, and can impart nuttiness and chocolate notes. Examples are 2,3,5-trimethylpyrazine (dark chocolate) and 2-acetyl pyrazine (popcorn). They may play a role in barrel ageing.

Detection methods
After swirling the wine, a short sniff of about one second should be enough to detect any methoxypyrazines present. The best way to familiarize yourself with the flavour is to smell a wine with a small quantity of bell pepper pyrazine added, and compare it with another glass with the same, untreated wine. There is another option: pour a Marlborough Sauvignon Blanc into a glass and pour a Chardonnay in a second glass. Sample the wines back and forth, and the green bell pepper character of the Sauvignon Blanc should become more apparent.

References

1. Roujou de Boubee, D. (2003). Research on 2-methoxy-3-isobutylpyrazine in grapes and wine. Amorim Academy Competition, 21pp.
2. Coetzee, C., & du Toit, W. J. (2012). A comprehensive review on Sauvignon blanc aroma with a focus on certain positive volatile thiols. Food Research International, 45(1), 287-298.
3. Coetzee, C. (2011). Oxygen and sulphur dioxide additions to Sauvignon blanc: effect on must and wine composition (Doctoral dissertation, Stellenbosch: University of Stellenbosch).
4. Pickering, G. J., & Botezatu, A. (2021). A Review of Ladybug Taint in Wine: Origins, Prevention, and Remediation. Molecules, 26(14), 4341.